Antenna configuration

ABSTRACT

Power is transmitted to an implanted receiving coil oriented such that an axis of the receiving coil is parallel to skin of a subject. A transmitting coil is in a housing, which is placed against the skin. A central axis of the transmitting coil is perpendicular to the skin. A portion of the transmitting coil is over the receiving coil. A first distance, from the axis of the transmitting coil to a center of the receiving coil, is greater than a second distance, from the axis of the transmitting coil to an inner edge of the portion of the transmitting coil. The first distance is less than a third distance, from the axis of the transmitting coil to an outer edge of the portion of the transmitting coil. Circuitry powers the implant by driving current through the transmitting coil that induces current in the receiving coil.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. Ser. No. 17/546,644 to Oron et al., filed Dec. 9, 2021, entitled “Antenna configuration,” which published as US 2022/0168580, and is a Continuation of U.S. Ser. No. 16/737,253 to Oron et al., filed Jan. 8, 2020, entitled “Antenna configuration,” which issued as U.S. Pat. No. 11,213,685 and is a Divisional of U.S. Ser. No. 15/621,433 to Oron et al., filed Jun. 13, 2017, entitled, “Antenna configuration,” which published as US 2018/0353764, and which is abandoned.

FIELD OF THE INVENTION

Applications of the present invention relate to transmitting power to an implanted medical device.

BACKGROUND

Electrical power can be transferred to a percutaneous medical implant by magnetic induction. A current flowing through a coil produces a magnetic field, which, in turn, will induce a current in a second coil, provided the second coil is in close enough proximity to the magnetic field and oriented such that the magnetic field is substantially parallel to the central longitudinal axis of the second coil. A coil inside a medical implant can therefore act as a receiving coil, while a coil outside a patient's body can act as a transmitting coil. A current can be driven through the transmitting coil in order to induce an induced current in the receiving coil, thereby powering the medical implant.

A CBS News article entitled “Migraine ‘smart’ patch tested to help ease pain,” by Steven Reinberg, describes a study performed at Rambam Medical Center in Haifa, Israel, under Dr. David Yarnitzky, chair of neurology at the Rambam Medical Center. The study tested an arm patch to be worn on the upper arm with “[r]ubber electrodes and a chip in the patch [to] produce electric impulses that block pain signals from reaching the brain,” in order to treat migraine pain.

A St. Jude Medical, Inc. fact sheet entitled “Peripheral nerve stimulation for intractable chronic migraine,” describes peripheral nerve stimulation as a treatment for chronic migraines. The fact sheet states that “Peripheral nerve stimulation (PNS) is a therapy that uses mild electrical pulses to stimulate the nerves of the peripheral nervous system. The peripheral nerves make up a network of nerves outside of the central nervous system. For example, the ulnar nerve in the arms and the sciatic nerve in the legs are part of the peripheral nervous system. The St. Jude Medical systems currently approved for PNS in select markets look and operate much like a cardiac pacemaker. However, instead of sending pulses to the heart, the pulses are carried to the occipital nerves, located in the back of the head. . . . Researchers believe that by delivering electrical pulses to these specific peripheral nerve fibers, PNS may influence the way the nerves communicate with the brain and provide an alternative to long-term drug therapy for the relief of chronic migraine.”

SUMMARY OF THE INVENTION

A method is described for transmitting power to a medical implant that includes a receiving coil. For some applications, a transmitting coil, disposed in a transmitting coil housing, is placed against skin of a subject such that a central longitudinal axis of the transmitting coil is substantially perpendicular to the skin. For some applications, the medical implant is implanted between an ankle and a knee of a leg of a subject, typically closer to the ankle than the knee. To increase efficiency of the power transfer while accommodating for limited space near the ankle, the transmitting coil is oriented with respect to the skin such that it is not centered over the receiving coil, but rather only a portion of the transmitting coil is disposed directly over the receiving coil. This orientation of the transmitting coil with respect to the receiving coil allows for powering the medical implant using only one transmitting coil. To transmit power to the medical implant, control circuitry is activated to drive a current through the transmitting coil that induces an induced current in the receiving coil.

Typically, the transmitting coil housing and the transmitting coil are flexible in order to comfortably conform to the shape of a limb of the subject. In the absence of any forces applied to the transmitting coil, the transmitting coil has a nominal resonance frequency. In order to accommodate for possible fluctuations in the resonance frequency of the transmitting coil due to the flexing, a sensor may be coupled to the control circuitry and configured to determine an extent of divergence of (a) the resonance frequency of the transmitting coil when the transmitting coil is flexed from (b) the nominal resonance frequency of the transmitting coil, occurring in the absence of any forces applied to the transmitting coil. The control circuitry is further configured to output a signal that controls one or more electrical components that are (a) coupled to the control circuitry and (b) configured to tune the resonance frequency of the transmitting coil in order to compensate for the fluctuations.

There is therefore provided, in accordance with some applications of the present invention, a method for transmitting power to a medical implant that includes a receiving coil that is oriented such that a longitudinal axis of the receiving coil is substantially parallel to skin of a subject, the method including: providing a transmitting coil disposed in a housing;

-   -   placing the housing against the skin such that:     -   (a) a central longitudinal axis of the transmitting coil is         substantially perpendicular to the skin,     -   (b) a portion of the transmitting coil is disposed over the         receiving coil,     -   (c) a first distance, from the central longitudinal axis of the         transmitting coil to a longitudinal center of the receiving         coil, is greater than a second distance, from the central         longitudinal axis of the transmitting coil to an inner edge of         the portion of the transmitting coil, and     -   (d) the first distance is less than a third distance, from the         central longitudinal axis of the transmitting coil to an outer         edge of the portion of the transmitting coil; and         activating control circuitry to power the medical implant by         driving a current through the transmitting coil that induces an         induced current in the receiving coil.

For some applications, placing includes identifying the subject as suffering from migraines or cluster headaches, and in response to the identifying, placing the housing on a leg of a subject such that:

(a) the transmitting coil is disposed between a knee and an angle of the leg, and

(b) the transmitting coil transmits power to a medical implant configured to stimulate a tibial nerve in the leg of the subject.

For some applications, placing includes placing the housing on a leg of the subject such that:

(a) the transmitting coil is disposed between a knee and an ankle of the leg, and

(b) (i) a portion of the transmitting coil that is disposed over the receiving coil is closer to the ankle than (ii) a portion of the transmitting coil that is disposed at 180 degrees from the portion of the transmitting coil that is disposed over the receiving coil, is to the ankle.

For some applications, placing includes placing the housing such that the first distance is 15-45 mm.

For some applications, placing includes placing the housing such that the second distance is less than 30 mm.

For some applications, placing includes placing the housing such that the third distance is 40-60 mm.

For some applications, placing includes placing the housing such that a difference between the third distance and the second distance is 30-40 mm.

For some applications, providing the transmitting coil includes providing a transmitting coil wherein a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is greater than 0.5.

For some applications, providing the transmitting coil includes providing a transmitting coil wherein a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is less than 1.5.

For some applications, providing the transmitting coil includes providing a transmitting coil wherein a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is between 0.5 and 1.5.

For some applications, providing the transmitting coil includes providing a transmitting coil wherein:

(a) a height of the transmitting coil measured along a longitudinal axis of the transmitting coil is 300-600 microns,

(b) an outer diameter of the transmitting coil is 100-140 mm, and

(c) a ratio of the outer diameter of the transmitting coil to the height of the transmitting coil is at least 150.

For some applications, placing includes placing the housing such that the transmitting coil is over a receiving coil, wherein:

(a) a longitudinal length of the receiving coil is 3-15 mm,

(b) an outer diameter of the receiving coil is 0.6-1.5 mm, and

(c) a ratio of the outer diameter of the receiving coil to the longitudinal length of the receiving coil is less than 0.5.

For some applications, activating the control circuitry includes activating the control circuitry to drive the current through the transmitting coil at a frequency of 1-20 MHz.

For some applications, placing includes placing the housing against the skin and subsequently sliding it along the skin until an indicator, coupled to the housing, indicates that the transmitting coil is in an acceptable position with respect to the receiving coil.

For some applications, providing the transmitting coil includes providing a transmitting coil wherein a cross-sectional area of a wire of the transmitting coil is rectangular, wherein the cross-section is taken perpendicular to a direction of current flow within the wire.

For some applications, providing the transmitting coil includes providing a transmitting coil that is elongated in a direction perpendicular to the central longitudinal axis of the receiving coil.

For some applications, providing the transmitting coil includes providing a planar coil disposed in a housing.

For some applications, providing the planar coil includes providing a planar coil including a plurality of layers.

For some applications, providing the planar coil includes providing a planar coil with a line spacing, of adjacent coplanar wires, of 0.25-3 mm.

For some applications, providing the planar coil includes providing a planar coil with a line width of 1-4 mm.

For some applications, providing the transmitting coil includes providing a transmitting coil wherein an average distance from a wire of the transmitting coil to the central longitudinal axis of the transmitting coil is less than two times a square root of a cross-sectional area of a central non-coiled region of the transmitting coil.

For some applications, providing includes providing a transmitting coil wherein an average distance from the wire of the transmitting coil to the central longitudinal axis of the transmitting coil is 0.6-1.5 times the square root of the cross-sectional area of the central non-coiled region of the transmitting coil.

There is further provided, in accordance with some applications of the present invention, apparatus including:

a medical implant, the medical implant including:

-   -   a receiving coil; and     -   a plurality of electrodes;

a transmitting coil, having wire disposed at all rotational locations about a central longitudinal axis of the transmitting coil, oriented such that:

-   -   (a) the central longitudinal axis of the transmitting coil is         substantially perpendicular to a central longitudinal axis of         the receiving coil,     -   (b) at one of the rotational locations, a line from the wire and         substantially parallel to the central longitudinal axis of the         transmitting coil intersects the receiving coil, and at 180         degrees from the rotational location a line from the wire and         substantially parallel to the central longitudinal axis of the         transmitting coil does not intersect the receiving coil,     -   (c) a first distance from the central longitudinal axis of the         transmitting coil to a longitudinal center of the receiving         coil, is greater than a second distance from the central         longitudinal axis of the transmitting coil to an inner edge of         the transmitting coil at the one of the rotational locations,         and     -   (d) the first distance is less than a third distance from the         central longitudinal axis of the transmitting coil to an outer         edge of the transmitting coil at the one of the rotational         locations; and

control circuitry configured to transmit power to the medical implant by driving a current through the transmitting coil that induces an induced current in the receiving coil.

For some applications, the control circuitry is configured to drive the current through the transmitting coil at a frequency of 1-20 MHz.

For some applications, the medical implant is configured to be implanted 1-5 cm below skin of a subject, and the control circuitry is configured to transmit the power, by driving the current through the transmitting coil that induces the induced current in the receiving coil, when the medical implant is implanted 1-5 cm below the skin.

For some applications, the receiving coil is a cylindrical coil including a ferrite core.

For some applications, the first distance is 15-45 mm.

For some applications, the second distance is less than 30 MM.

For some applications, the third distance is 40-60 mm.

For some applications, a difference between the third distance and the second distance is 30-40 mm.

For some applications, a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is greater than 0.5.

For some applications, a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is less than 1.5.

For some applications, a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is between 0.5 and 1.5.

For some applications:

(a) a height of the transmitting coil measured along a longitudinal axis of the transmitting coil is 300-600 microns,

(b) an outer diameter of the transmitting coil is 100-140 mm, and

(c) a ratio of the outer diameter of the transmitting coil to the height of the transmitting coil is at least 150.

For some applications:

(a) a longitudinal length of the receiving coil is 3-15 mm,

(b) an outer diameter of the receiving coil is 0.6-1.5 mm, and

(c) a ratio of the outer diameter of the receiving coil to the longitudinal length of the receiving coil is less than 0.5.

For some applications:

(a) a first ratio, of the outer diameter of the transmitting coil to a height of the transmitting coil measured along a longitudinal axis of the transmitting coil, is at least 150,

(b) a second ratio, of the outer diameter of the receiving coil to the longitudinal length of the receiving coil, is less than 0.5, and

(c) a ratio of the first ratio to the second ratio is at least 300.

For some applications, the transmitting coil has between 4 and 10 turns.

For some applications, the receiving coil has between 10 and 40 turns.

For some applications, the medical implant is configured to send a signal to the control circuitry upon receiving the transmitted power.

For some applications, a cross-sectional area of a wire of the transmitting coil is rectangular, the cross-section being taken perpendicular to a direction of current flow within the wire.

For some applications, the transmitting coil is elongated in a direction perpendicular to the central longitudinal axis of the receiving coil.

For some applications, a length of the receiving coil is 3-15 mm.

For some applications, the medical implant includes a housing having a length of 30-45 mm and the receiving coil is disposed in within the housing.

For some applications, the apparatus further includes an indicator, and the control circuitry is configured to activate the indicator upon the transmitting coil being in an acceptable position with respect to the receiving coil.

For some applications, the control circuitry is configured to detect interference with its output signal and to activate the indicator upon the detection of the interference.

For some applications, the control circuitry is configured to activate the indicator again, upon the transmitting coil no longer being in correct position with respect to the receiving coil.

For some applications, the control circuitry is configured to ascertain an indication of an efficiency of the power transfer between the transmitting coil and the receiving coil, and to activate the indicator according to the ascertaining.

For some applications, the control circuitry is configured to measure a loss of power in the transmitting coil, the loss of power being indicative of the efficiency of the power transfer.

For some applications, the transmitting coil is a planar coil.

For some applications, a line width of the transmitting coil is 1-4 mm.

For some applications, the planar coil includes a plurality of layers.

For some applications, a line spacing of adjacent coplanar wires in the transmitting coil is 0.25-3 mm.

For some applications, the apparatus further includes a flexible printed circuit board (PCB), and the transmitting coil includes two planar layers disposed on either side of the flexible PCB.

For some applications, a height of each layer measured along a longitudinal axis of the transmitting coil is 15-100 microns.

For some applications, a height of the flexible PCB measured along a longitudinal axis of the transmitting coil is 100-200 microns.

For some applications, respective wires of the two layers are conductively connected to each other at at least one location along each turn of the transmitting coil.

For some applications, the apparatus further includes at least one capacitor, coupled to the transmitting coil at at least one location along at least one turn of the transmitting coil.

For some applications, the capacitor is electrically coupled to both of the two layers.

For some applications, the apparatus further includes a plurality of capacitors coupled to the transmitting coil such that at least one capacitor is coupled to the transmitting coil at at least one location along each turn of the transmitting coil.

For some applications, each of the capacitors is electrically coupled to both of the two layers.

For some applications, an insulating cover is coupled to both layers of the transmitting coil disposed on the flexible PCB.

For some applications, an average distance from a wire of the transmitting coil to the central longitudinal axis of the transmitting coil is less than two times a square root of a cross-sectional area of a central non-coiled region of the transmitting coil.

For some applications, the average distance from the wire of the transmitting coil to the central longitudinal axis of the transmitting coil is 0.6-1.5 times the square root of the cross-sectional area of the central non-coiled region of the transmitting coil.

There is further provided, in accordance with some applications of the present invention, apparatus for use with a medical implant that includes a receiving coil, the apparatus including:

a flexible housing configured to be placed against skin of a subject;

a flexible transmitting coil disposed in the housing;

control circuitry configured to transmit power to the medical implant by driving a current through the transmitting coil that induces an induced current in the receiving coil;

a sensor coupled to the control circuitry, the sensor configured to determine an extent of divergence of (a) a resonance frequency of the transmitting coil when the transmitting coil is flexed from (b) a nominal resonance frequency of the transmitting coil, occurring in the absence of any forces applied to the transmitting coil, and configured to output a signal according to the determination; and

one or more electrical components, coupled to the control circuitry and configured to tune the resonance frequency of the transmitting coil in response to the determination of the sensor.

For some applications, the control circuitry is configured to set the frequency of the current output by the control circuitry to be between 1 and 20 MHz.

For some applications, the flexible transmitting coil is configured to flex such that it can substantially conform to a lateral wall of a cylinder having a diameter between 8 and 50 cm.

For some applications, the sensor includes a phase detector, configured to (a) determine a phase difference between the phase of the current output by the control circuitry, and the phase of either a current or a voltage on at least one component of the transmitting coil, the phase difference being due to flexing of the transmitting coil, and (b) output a signal according to the determination.

For some applications, the control circuitry includes a feedback calculator configured to:

(a) receive the signal output by the phase detector,

(b) determine, according to the signal output by the phase detector, a necessary change in the resonance frequency of the transmitting coil, in order to reduce the extent of divergence of (a) the resonance frequency of the transmitting coil when the transmitting coil is flexed from (b) the nominal resonance frequency of the transmitting coil, and

(c) output a signal to the one or more electrical components, according to the determination.

For some applications, the sensor is configured to:

(a) measure a parameter that is indicative of the frequency of the current output by the control circuitry and the resonance frequency of the transmitting coil,

(b) look up at least one value in a look-up table with respect to the measured parameter, and

(c) output a signal to the one or more electrical components based on the looked-up value.

For some applications, the control circuitry is configured such that the measured parameter is a level of power output by the transmitting coil.

For some applications, at least one of the one or more electrical components is a variable inductor, the control circuitry is configured to vary an inductance of the variable inductor according to the signal output by the sensor, and the resonance frequency of the transmitting coil varies according to the variation of the inductance of the variable inductor.

For some applications, at least one of the one or more electrical components is a variable capacitor, the control circuitry is configured to vary a capacitance of the variable capacitor according to the signal output by the sensor, and the resonance frequency of the transmitting coil varies according to the variation of the capacitance of the variable capacitor.

For some applications, the apparatus further includes a plurality of switches, each switch coupled to a respective one of the electrical components.

For some applications, the control circuitry is configured to tune the resonance frequency of the transmitting coil, according to the signal output by the sensor, by activating at least one of the plurality of switches to facilitate or inhibit current flow through the respective electrical component.

For some applications, the control circuitry is configured to dither the resonance frequency of the transmitting coil by repeatedly activating and deactivating the at least one of the plurality of switches to facilitate or inhibit current flow through the respective electrical component.

For some applications, at least one of the plurality of switches is configured to be manually operated and the remaining switches are configured to be operated by the control circuitry, wherein (a) the electrical component coupled to the manually-operated switch is configured to vary the resonance frequency of the transmitting coil by more than (b) the electrical components coupled to the switches operated by the control circuitry are configured to vary the resonance frequency of the transmitting coil.

For some applications, the one or more electrical components is a plurality of inductors, coupled in series.

For some applications, the plurality of inductors includes 3-9 inductors.

For some applications, a first one of the inductors has an inductance of 1.5-2.5 times an inductance of another one of the inductors.

For some applications, the inductance of the first one of the inductors is twice the inductance of the other one of the inductors.

For some applications, each one of at least half of the inductors has an inductance which is twice an inductance of another one of the inductors.

For some applications, the control circuitry is configured such that when the extent of divergence of (a) the resonance frequency of the transmitting coil when the transmitting coil is flexed from (b) the nominal resonance frequency of the transmitting coil is reduced, current is allowed to pass through at least one of the inductors and current is inhibited from passing through at least another one of the inductors.

For some applications, the one or more electrical components is a plurality of capacitors coupled in parallel.

For some applications, the plurality of capacitors includes 4 to 10 capacitors.

For some applications, a first one of the capacitors has a capacitance of 1.5-2.5 times a capacitance of another one of the capacitors.

For some applications, the capacitance of the first one of the capacitors is twice the capacitance of the other one of the capacitors.

For some applications, each one of at least half of the capacitors has a capacitance that is twice a capacitance of another one of the capacitors.

For some applications, the control circuitry is configured such that when the extent of divergence of (a) the resonance frequency of the transmitting coil when the transmitting coil is flexed from (b) the nominal resonance frequency of the transmitting coil is reduced, current is allowed to pass through at least one of the capacitors and current is inhibited from passing through at least another one of the capacitors.

For some applications, the one or more electrical components is a plurality of electrical components including inductors, coupled in series, and capacitors, coupled in parallel.

For some applications, a first one of the inductors has an inductance of 1.5-2.5 times an inductance of another one of the inductors.

For some applications, the inductance of the first one of the inductors is twice the inductance of the other one of the inductors.

For some applications, each one of at least half of the inductors has an inductance that is twice an inductance of another one of the inductors.

For some applications, a first one of the capacitors has a capacitance of 1.5-2.5 times a capacitance of another one of the capacitors.

For some applications, the capacitance of the first one of the capacitors is twice the capacitance of the other one of the capacitors.

For some applications, each one of at least half of the capacitors has a capacitance that is twice a capacitance of another one of the capacitors.

For some applications, the control circuitry is configured such that when the extent of divergence of (a) the resonance frequency of the transmitting coil when the transmitting coil is flexed from (b) the nominal resonance frequency of the transmitting coil is reduced, current is allowed to pass through at least one of the electrical components and current is inhibited from passing through at least another one of the electrical components.

For some applications:

the control circuitry is configured to activate the switches by applying a respective voltage of 30-300 volts to each switch, the switches include transistors, acting as diodes, having respective capacitances that are dependent on the respective voltage applied to each switch.

For some applications, the control circuitry is configured to apply the respective voltages to the respective switches at a voltage of 50-200 volts.

For some applications:

the control circuitry is configured to activate the switches by applying a respective voltage of 30-300 volts to each switch, and

the switches include transistors, which behave in their off states as variable capacitors, having respective capacitances that are dependent on the respective voltage applied to each switch.

For some applications, the control circuitry is configured to apply the respective voltages to the respective switches at a voltage of 50-200 volts.

For some applications, the apparatus further includes the medical implant.

There is further provided, in accordance with some applications of the present invention, a method for treating a subject suffering from migraines or cluster headaches, the method including:

identifying the subject as suffering from migraines or cluster headaches; and

in response to the identifying, powering a medical implant to stimulate a tibial nerve in a leg of the subject.

The present invention will be more fully understood from the following detailed description of applications thereof, taken together with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B are schematic illustrations of a medical implant comprising a receiving coil under skin of a subject and a transmitting coil disposed in a housing that is placed against the skin, in accordance with some applications of the present invention;

FIGS. 2A-B are schematic illustrations of a cross-sectional view of the receiving coil disposed in the medical implant and the transmitting coil disposed in the housing against the skin, in accordance with some applications of the present invention;

FIG. 3 is a schematic illustration of the orientation of the transmitting coil with respect to the receiving coil, in accordance with some applications of the present invention;

FIG. 4 is a schematic illustration of a top view and a cross-section of the transmitting coil, in accordance with some applications of the present invention;

FIGS. 5A-B are schematic illustrations of a cross-sectional view of the transmitting coil and a top view of the transmitting coil, in accordance with some applications of the present invention;

FIGS. 6-7 are schematic illustrations of the transmitting coil in the housing being flexed to conform to a curve of a limb of the subject, in accordance with some applications of the present invention;

FIGS. 8-13 are schematic illustrations of control circuitry of the transmitting coil, in accordance with some applications of the present invention;

FIG. 14 is a circuit diagram of the control circuitry of the transmitting coil, in accordance with some applications of the present invention; and

FIG. 15 is a graph showing rate of change of capacitance versus drain-to-source voltage change of a switch coupled to the control circuitry, in accordance with some applications of the present invention.

DETAILED DESCRIPTION

Reference is made to FIGS. 1A-B, which are schematic illustrations of a transmitting coil 20, disposed in a transmitting coil housing 22 that is placed against skin 28 of a subject, and a medical implant 23, under skin 28 of a limb 30 of a subject, comprising a receiving coil 24 that is disposed in a receiving coil housing 26, in accordance with some applications of the present invention. Typically, receiving coil housing 26 is oriented such that a central longitudinal axis 32 of receiving coil 24 is substantially parallel to skin 28. Transmitting coil housing 22 of transmitting coil 20 is typically placed against skin 28 and oriented such that a central longitudinal axis 34 (FIG. 2) of transmitting coil 20 is substantially perpendicular to skin 28. Power is transmitted to medical implant 23 by activating control circuitry 36 (FIG. 1B), coupled to transmitting coil housing 22, to drive a current through transmitting coil 20, for example at a frequency of 1-20 MHz, e.g., a fixed frequency of 6.78 or 13.56 MHz. For some applications, lower frequencies such as 0.1-0.5 MHz may also be used. A magnetic field, for example magnetic field 52 (FIG. 2A), generated by the current in transmitting coil 20, induces an induced current in receiving coil 24.

As used in the present application, including in the claims, a “central longitudinal axis” of an elongate structure is the set of all centroids of transverse cross-sectional sections of the structure along the structure. Thus, the cross-sectional sections are locally perpendicular to the central longitudinal axis, which runs along the structure. (If the structure is circular in cross-section, the centroids correspond with the centers of the circular cross-sectional sections.)

As used in the present application, including in the claims, substantially parallel elements are to be understood as having an angle between them that is less than 10 degrees. For some applications, substantially parallel elements have an angle between them that is less than 5 degrees.

As used in the present application, including in the claims, substantially perpendicular elements are to be understood as having an angle between them that is at least 85 degrees and/or less than 95 degrees.

Reference is now made to FIGS. 2A-B, which are schematic illustrations of an orientation of transmitting coil housing 22, and thereby transmitting coil 20, with respect to receiving coil 24, in accordance with some applications of the present invention. For some applications, medical implant 23 is implanted between a knee and an ankle of a subject, closer to the ankle than to the knee, such as is shown in FIGS. 1A-B. Typically, a doctor will implant the medical implant 2-10 cm away from the medial malleolus. Inside medical implant 23, receiving coil 24 is disposed closer to an ankle-side 84 (FIG. 2A) of the medical implant than it is to a knee-side 86 (FIG. 2A) of the medical implant. Space on skin 28 near the ankle however is limited due to the subject's ankle bone and shoe. Efficiency of the power transfer can be improved by ensuring that magnetic fields, e.g., magnetic field 52, generated by the current in transmitting coil 20 are substantially parallel to receiving coil 24 in the vicinity of receiving coil 24. Magnetic fields that are substantially parallel to receiving coil 24, but not in the vicinity of receiving coil 24, e.g., magnetic field 53, do not substantially affect the power transfer.

Therefore, taking into account the limited space available near the ankle, the desired orientation of the generated magnetic fields is accomplished by placing transmitting coil housing 22 against skin 28 such that transmitting coil 20, having a wire disposed at all rotational locations about central longitudinal axis 34, is not centered over receiving coil 24. Rather, only a portion 50 (FIG. 1A) of transmitting coil 20 is disposed over receiving coil 24, such that at one of the rotational locations about central longitudinal axis 34 of transmitting coil 20, a line 38 extending from the wire and substantially parallel to central longitudinal axis 34 of transmitting coil 20 intersects receiving coil 24, and at 180 degrees from the rotational location, a line 40 extending from the wire and substantially parallel to central longitudinal axis 34 of transmitting coil 20 does not intersect receiving coil 24. This orientation of transmitting coil 20, further described hereinbelow, allows for the use of only one transmitting coil 20 to power medical implant 23. Typically, (a) portion 50 of transmitting coil 20, that is disposed over receiving coil 24, is closer to the ankle than (b) a portion 51 of transmitting coil 20, that is not disposed over receiving coil 24, e.g., disposed at 180 degrees from portion 50, is to the ankle (for example as shown in FIG. 1A).

For some applications, medical implant 23 is implanted on a leg between the knee and the ankle, as described hereinabove, in order to treat patients suffering from migraines or cluster headaches using tibial nerve stimulation. Transmitting coil 20 powers medical implant 23 in order to provide neural stimulation to the tibial nerve, for example at a repetition rate of 10-60 Hz. Similarly to over-stimulation of the ulnar nerve for treatment of migraines, over-stimulation of the tibial nerve may cause paresthesia in the active pain centers in the brain, thereby reducing the pain of the migraine or cluster headache.

Reference is now made to FIG. 3, which is a schematic illustration of transmitting coil 20 disposed in transmitting coil housing 22 and receiving coil 24 disposed in receiving coil housing 26, in accordance with some applications of the present invention. Typically, transmitting coil housing 22 is placed such that (a) a first distance D1, from central longitudinal axis 34 of transmitting coil 20 to a longitudinal center 42 of receiving coil 24, is 15-45 mm, (b) a second distance D2, from central longitudinal axis 34 of transmitting coil 20 to an inner edge 44 of portion 50 of transmitting coil 20 that is disposed over receiving coil 24, is less than 30 mm, and (c) a third distance D3, from central longitudinal axis 34 of transmitting coil 20 to an outer edge 46 of portion 50 of transmitting coil 20 that is disposed over receiving coil 24, is 40-60 mm. Typically, (a) first distance D1 is greater than second distance D2 and less than third distance D3, and (b) a difference between second distance D2 and third distance D3 is 30-40 mm. The difference between third distance D3 and second distance D2 is referred to hereinbelow as width W of transmitting coil 20.

In order to further improve the efficiency of the power transfer, transmitting coil 20 is typically elongated in a direction perpendicular to central longitudinal axis 32 of receiving coil 24 thus increasing a distance between central longitudinal axis 34 and a wire of transmitting coil 20. Therefore, magnetic fields generated by the current in transmitting coil 20, e.g., magnetic field 54, that are not substantially parallel to receiving coil 24, are farther away from receiving coil 24 thereby they have less of an effect on the induced current in receiving coil 24. For some applications, an average distance D10 (FIG. 4) from the wire of transmitting coil 20 to central longitudinal axis 34 of transmitting coil 20 is less than two times, e.g., 0.6-1.5 times, a square root of a cross-sectional area of a central non-coiled region 56 of transmitting coil 20.

Efficiency of the power transfer is also affected by a depth of implantation of medical implant 23. Typically, medical implant 23 is implanted at a depth D14 (FIG. 2B) of 1-5 cm below skin 28. As used in the present application, including in the claims, the depth of medical implant 23 is the distance from skin 28 to central longitudinal axis 32 of receiving coil 24 measured substantially normal to the skin.

Reference is again made to FIG. 1B. In some applications, an indicator 48 is coupled to transmitting coil housing 22. Control circuitry 36 is configured to activate indicator 48 upon transmitting coil 20 being in an acceptable position with respect to receiving coil 24. For example, indicator 48 may be a visual indicator, an audible indicator, or a vibrator. Typically, indicator 48 is configured to indicate an acceptable position of transmitting coil 20 when the efficiency of the energy transmission between transmitting coil 20 and receiving coil 24 is above a threshold that is at least 85% of the maximum efficiency possible for the patient. For some applications, the maximum efficiency of the power transfer is approximately 5%.

For some applications, control circuitry 36 is able to detect when transmitting coil 20 is in an acceptable position by outputting a signal and subsequently detecting an interference, caused by receiving coil 24, with the signal. Upon detection of the interference, control circuitry 36 activates indicator 48.

Alternatively or additionally, control circuitry 36 is able to ascertain an indication of the efficiency of the energy transmission between transmitting coil 20 and receiving coil 24, and indicator 48 is configured to have a range of indications that are respectively representative of the efficiency ascertained by control circuitry 36. For some applications, the indication of the efficiency is a measurement of power loss in transmitting coil 20. Power loss in transmitting coil 20 may include one or more of the following: (a) power losses that do not appreciably change with the positioning of the transmitting coil, such as losses due to unavoidable resistance of transmitting coil 20 and other losses in the transmitting electronics, and (b) losses in the power transmitted to medical implant 23 which depend on the relative positioning of transmitting coil 20 and receiving coil 24, such as absorption of power in the tissue and surrounding structures. Thus, monitoring the power loss in transmitting coil 20 may facilitate proper positioning of transmitting coil 20 in relation to medical implant 23.

Alternatively or additionally, medical implant 23 is configured to send an output signal to control circuitry 36 upon receiving transmitted power from transmitting coil 20. This output signal may include data indicative of the power received by receiving coil 24 in medical implant 23. Control circuitry 36 receives the data indicative of the power received by receiving coil 24 in medical implant 23, and by comparing it to the power transmitted by transmitting coil 20, determines a parameter indicative of the efficiency of the power transmission. This parameter may be used to indicate to the user: a) if the efficiency is within a range of acceptable values; and b) if repositioning transmitting coil housing 22 has caused an increase or decrease in the power transmission. The indication may be used by a healthcare provider, during an initial training session, to train the patient or family member to correctly position transmitting coil housing 22. Similarly, the indication may be used by the patient or family member each time transmitting coil housing 22 has to be placed on the patient or repositioned. For some applications, the output signal from medical implant 23, indicative of the power received by receiving coil 24, is sent only when needed. For example, the output signal from medical implant 23 may be sent (a) when medical implant 23 is powered-up, (b) during positioning of transmitting coil 20, or (c) when the power received by receiving coil 24 in medical implant 23 is changed unexpectedly, indicating a possible movement of transmitting coil 20 relative to receiving coil 24.

Transmitting coil housing 22 can be positioned on skin 28 by placing housing 22 against skin 28 and subsequently sliding transmitting coil housing 22 along skin 28 until indicator 48 indicates that transmitting coil 20 is in an acceptable position with respect to receiving coil 24. In some applications, control circuitry 36 is further configured to activate indicator 48 again upon transmitting coil 20 no longer being in an acceptable position with respect to receiving coil 24.

Reference is now made to FIG. 4, which is a schematic illustration of transmitting coil 20 in accordance with some applications of the present invention. Typically, transmitting coil 20 is a planar coil having between 4 and 10 turns, e.g., 8 turns. A line spacing D9 of adjacent coplanar wires in transmitting coil 20 is typically 0.25-3 mm, e.g., 2 mm, and a line width D8 of the wires in transmitting coil 20 is typically 1-4 mm, e.g., 2 mm. For some applications, transmitting coil 20 comprises a plurality of planar layers, e.g., two planar layers.

One or more dimensions of transmitting coil 20 that highlight the planar properties of transmitting coil 20 are as follows:

-   -   a height D5 (FIG. 2B) of transmitting coil 20, measured along         central longitudinal axis 34 of transmitting coil 20 when         transmitting coil 20 is laid flat, i.e. the thickness of         transmitting coil 20, is at least 300 and/or less than 600         microns;     -   an outer diameter D6 (FIG. 3) of transmitting coil 20 is at         least 100 mm and/or less than 140 mm;     -   a ratio of outer diameter D6 of transmitting coil 20 to height         D5 of transmitting coil 20 is at least 150.

As used in the present application, including in the claims, outer diameter D6 of transmitting coil 20 is the largest dimension of transmitting coil 20 from one side of the coil to the other, measured perpendicular to central longitudinal axis 34 of transmitting coil 20.

Typically, a cross-sectional area 108 of the wire of transmitting coil 20 is rectangular when the cross-section, e.g., cross-section A-A shown in FIG. 4, is taken perpendicular to a direction of current flow within the wire.

Typically, receiving coil 24 is a cylindrical coil having 10-40 turns, e.g., 20 turns, and comprising a ferrite core. For some applications, one or more dimensions of receiving coil 24 are as follows:

-   -   a longitudinal length D4 (FIG. 3) of receiving coil 24 is at         least 3 mm and/or less than 15 mm;     -   an outer diameter D7 of receiving coil 24 (FIG. 2B) is at least         0.6 mm and/or less than 1.5 mm; and/or     -   a ratio of outer diameter D7 of receiving coil 24 to         longitudinal length D4 of receiving coil 24 is less than 0.5.

Typically, receiving coil housing 26 is longitudinally longer than receiving coil 24, to accommodate for control circuitry disposed within medical implant 23. For some applications, a longitudinal length D11 of receiving coil housing 26 is at least 30 mm and/or less than 45 mm. Medical implant 23 may also comprise a plurality of electrodes.

For some applications, some dimensional relationships between transmitting coil 20 and receiving coil 24 are expressed according to a set of one or more of the following options:

-   -   (a) a first ratio, of outer diameter D6 (FIG. 3) of transmitting         coil 20 to height D5 (FIG. 2B) of transmitting coil 20 is at         least 150, (b) a second ratio, of outer diameter D7 (FIG. 2B) of         receiving coil 24 to longitudinal length D4 (FIG. 3) of         receiving coil 24 is less than 0.5, and (c) a ratio of the first         ratio to the second ratio is at least 300;     -   a ratio of width W of transmitting coil 20 to longitudinal         length D4 (FIG. 3) of receiving coil 24 is greater than 0.5;     -   a ratio of width W of transmitting coil 20 to longitudinal         length D4 of receiving coil 24 is less than 1.5; and/or     -   a ratio of width W of transmitting coil 20 to longitudinal         length D4 of receiving coil 24 is at least 0.5 and/or less than         1.5.

Reference is now made to FIGS. 5A-B, which are schematic illustrations of transmitting coil 20, in accordance with some applications of the present invention. FIG. 5A depicts schematic cross-sectional views of several locations of transmitting coil 20, in accordance with some applications of the present invention. For some applications, transmitting coil 20 comprises two planar layers 94 and 96 disposed on either side of a flexible printed circuit board (PCB) 98. A height D12 of each planar layer 94 and 96, measured along longitudinal axis 34 of transmitting coil 20, is 15-100 microns, e.g., 35 or 70 microns, and a thickness D13 of flexible PCB 98 is 100-200 microns, e.g., 150 microns. For some applications, at least once along each turn of transmitting coil 20 the two planar layers 94 and 96 are conductively connected to each other, such that current may flow from one layer to the other. For example, a via 100 filled with solder may be used to conductively connect the two planar layers 94 and 96.

Additionally, a capacitor 102 is coupled to transmitting coil 20 at at least one location along at least one turn of transmitting coil 20. Typically, capacitor 102 is attached to an exposed pad 92 of one of planar layers 94 or 96. For some applications, as seen in FIG. 5A, capacitor 102 is electrically coupled to both planar layers 94 and 96 by being coupled to two or more vias 100 in pad 92. For some applications, as seen in FIG. 5B, capacitor 102 is directly soldered to pad 92. For some applications, a plurality of capacitors 102 are coupled to transmitting coil 20 such that at least one capacitor 102 is coupled to transmitting coil 20 at at least one location along each turn of transmitting coil 20.

Typically, an insulating cover 104 is coupled, e.g., glued, to both planar layers 94 and 96 of transmitting coil 20 on flexible PCB 98. For some applications, a thickness D17 of a layer of glue 106 between cover 104 and each planar layer 94 and 96 is 15-50 microns. For some applications, a thickness D18 of cover 104 is 15-100 microns.

FIG. 5A shows both a cross-section and a top-view of transmitting coil 20. In the top-view, one planar layer 94 can be seen on flexible PCB 98, with one capacitor 102 coupled to each turn of transmitting coil 20. A plurality of solder-filled vias 100 are coupled to each turn of transmitting coil 20 to conductively connect planar layer 94 to planar layer 96, which is coupled to the other side of flexible PCB 98 and not visible in this figure. For some applications, vias 100 are positioned at the corners of each turn of transmitting coil 20, and on either side of each capacitor 102, as shown in FIG. 5A. Alternatively or additionally a plurality of vias 100, e.g., 2-30 vias 100 may be positioned anywhere along each turn of transmitting coil 20.

Reference is now made to FIGS. 6-7, which are schematic illustrations of transmitting coil housing 22, comprising transmitting coil 20, placed against skin 28 of limb 30 of the subject, in accordance with some applications of the present invention. Typically, in order to allow comfortable placement of transmitting housing 22 against limb 30, transmitting coil housing 22 and transmitting coil 20 are configured to be flexible such that they can substantially conform to a lateral wall of cylinders having diameters that range between a diameter D15 (FIG. 6) of 8 cm, e.g., a wrist, and a diameter D16 (FIG. 7) of 50 cm, e.g., a torso or obese upper leg. The flexing of transmitting coil 20, however, may cause the resonance frequency of transmitting coil 20 to fluctuate rather than remain at a nominal resonance frequency that occurs in the absence of any forces applied to transmitting coil 20 and is near the frequency of the current output by control circuitry 36.

Reference is now made to FIGS. 8-13, which are circuit diagrams of control circuitry 36, in accordance with some applications of the present invention. Portion 21 of control circuitry 36, as shown in FIGS. 8-13, is a model of transmitting coil 20 as shown in FIGS. 1-7. For some applications, a sensor 58 is coupled to control circuitry 36. Sensor 58 is coupled to control circuitry 36 and is configured to (a) determine an extent of divergence of (i) the resonance frequency of transmitting coil when transmitting coil 20 is flexed from (ii) the nominal resonance frequency of transmitting coil 20, and (b) subsequently output a signal to one or more electrical components that are coupled to control circuitry 36 and configured to tune the resonance frequency of transmitting coil 20 in response to the determination of sensor 58.

For some applications, sensor 58 comprises a phase detector 60 and a feedback calculator 62 (for example, as shown in FIGS. 8-12). Phase detector 60 is configured to (a) determine a phase difference between the phase of the current output by signal generator 64 and the phase of either a current or a voltage on at least one component of transmitting coil 20, and (b) output a signal to feedback calculator 62 according to the determination. After receiving the signal from phase detector 60, feedback calculator 62 (a) determines a necessary change in the resonance frequency of transmitting coil 20 that will reduce the extent of divergence of (i) the resonance frequency of transmitting coil 20 when transmitting coil 20 is flexed from (ii) the nominal resonance frequency of transmitting coil 20 and (b) outputs a signal to the electrical components according to the determination. Dashed lines 88 (FIGS. 8-12 represent feedback calculator 62 controlling each respective switch 74.

For some applications (e.g., as shown in FIG. 13), sensor 58 does not comprise phase detector 60 and feedback calculator 62. Rather, sensor 58 is configured to (a) measure a parameter that is indicative of both the frequency output by signal generator 64 and the resonance frequency of transmitting coil 20, e.g., by measuring the power output of transmitting coil 20, (b) look up at least one value in a look-up table with respect to the measured parameter, and (c) output a signal to the electrical components based on the looked-up value. Dashed line 90 (FIG. 13) represents sensor 58 controlling each respective switch 74.

For some applications, at least one of the electrical components is a variable inductor 66 (FIG. 8), whose inductance is varied according to the signal output by sensor 58. Variation of the inductance of variable inductor 66, in turn, cause variations in the resonance frequency of transmitting coil 20.

For some applications, at least one of the electrical components is a variable capacitor 68 (FIG. 9), whose capacitance is varied according to the signal output by sensor 58. Variation of the capacitance of variable capacitor 68, in turn, cause variations in the resonance frequency of transmitting coil 20.

For some applications, the one or more electrical components is a (a) a plurality of inductors 70, e.g., 3-9 inductors 70, coupled in series (FIG. 10), (b) a plurality of capacitors 72, e.g., 4-10 capacitors 72, coupled in parallel (FIG. 11), or (c) a combination of inductors 70, coupled in series, and capacitors 72, coupled in parallel (FIG. 12). When a plurality of inductors are used, typically a first one of inductors 70 has an inductance of 1.5-2.5 times, e.g., 2 times, an inductance of another one of inductors 70, and/or each one of at least half of inductors 70 has an inductance that is twice an inductance of another one of inductors 70. For example, 9 inductors 70 may have respective inductances of 2, 4, 8, 16, 32, 64, 128, 256, and 512 (arbitrary units). Similar sequencing may be used for a plurality of capacitors 72. For example, 10 capacitors 72 may have respective capacitances of 2, 4, 8, 16, 32, 64, 128, 256, 512, and 1024 (arbitrary units).

Typically, control circuitry 36 tunes the resonance frequency of transmitting coil 20, according to the signal output by sensor 58, by activating and/or deactivating at least one of a plurality of switches 74, each switch 74 being coupled to a respective one of the electrical components, in order to facilitate or inhibit current flow through the respective electrical component. In order to easily be able to increase and decrease the resonance frequency of transmitting coil 20, as necessary according to the signal output by sensor 58, control circuitry 36 is configured such that, when the extent of divergence of (a) the resonance frequency of transmitting coil 20 when transmitting coil 20 is flexed from (b) the nominal resonance frequency of transmitting coil 20 is reduced, at least one of switches 74 is activated, allowing current to flow through a respective electrical component, and at least another switch 74 is deactivated, inhibiting current from flowing through another respective electrical component. For some applications, control circuitry 36 is configured to dither the resonance frequency of transmitting coil 20 by repeatedly activating and deactivating at least one of switches 74 to alternatingly facilitate and inhibit current flow through a respective electrical component.

For some applications, a wider range of variation of the resonance frequency of transmitting coil 20 may be achieved by having at least one electrical component (a) configured to vary the resonance frequency of transmitting coil 20 by more than the remaining electrical components are configured to vary the resonance frequency of transmitting coil 20 and (b) coupled to a manually-operated switch. The manually-operated switch may be activated and/or deactivated by a user to provide gross tuning of the resonance frequency of transmitting coil 20 and the remaining switches 74 activated and/or deactivated by control circuitry 36 to provide fine tuning of the resonance frequency of transmitting coil 20.

Reference is now made to FIG. 14, which is a circuit diagram of control circuitry 36, in accordance with some applications of the present invention. For some applications, switches 74 comprise transistors 76 (e.g., field effect transistors, e.g., MOSFETs, as shown in FIG. 14) that behave, in their off state, as either diodes or variable capacitors, such that each switch 74 has a respective parasitic capacitance that depends on a respective voltage applied to each switch 74.

Reference is now made to FIG. 15, which is a graph showing rate of capacitance change versus drain-to-source voltage change of a switch, such as a switch 74 in control circuitry 36, in accordance with some applications of the present invention. Curve 78 of the graph shows (a) how the rate of capacitance change of a switch, such as switch 74, is significantly decreased when the switch is activated by an alternating current (AC) or, as shown in FIG. 14, a DC voltage, of over 50 volts, and (b) how the output capacitance of a switch, such as switch 74, significantly decreases as the drain-to-source voltage is increased from 0-50 volts. As shown by arrows 80 and 82, in order to reduce an effect that the respective parasitic capacitances of respective switches 74 may have on the resonance frequency of transmitting coil 20, control circuitry 36 is configured to activate switches 74 by applying a respective AC or, as shown in FIG. 14, DC voltage of 30-300 volts (arrow 80), e.g., 50-200 volts (arrow 82), to each switch 74, thereby reducing the output capacitance of each switch 74, as well as reducing the variation in output capacitance of each switch 74 over the duration of the AC voltage cycle or over the duration of the application of the DC voltage.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description. 

1. A method for transmitting power to a medical implant that comprises a receiving coil that is oriented such that a longitudinal axis of the receiving coil is substantially parallel to skin of a subject, the method comprising: providing a transmitting coil disposed in a housing; placing the housing against the skin such that: (a) a central longitudinal axis of the transmitting coil is substantially perpendicular to the skin, (b) a portion of the transmitting coil is disposed over the receiving coil, (c) a first distance, from the central longitudinal axis of the transmitting coil to a longitudinal center of the receiving coil, is greater than a second distance, from the central longitudinal axis of the transmitting coil to an inner edge of the portion of the transmitting coil, and (d) the first distance is less than a third distance, from the central longitudinal axis of the transmitting coil to an outer edge of the portion of the transmitting coil; and activating control circuitry to power the medical implant by driving a current through the transmitting coil that induces an induced current in the receiving coil.
 2. The method according to claim 1, wherein placing comprises identifying the subject as suffering from migraines or cluster headaches, and in response to the identifying, placing the housing on a leg of a subject such that: (a) the transmitting coil is disposed between a knee and an angle of the leg, and (b) the transmitting coil transmits power to a medical implant configured to stimulate a tibial nerve in the leg of the subject.
 3. The method according to claim 1, wherein placing comprises placing the housing on a leg of the subject such that: (a) the transmitting coil is disposed between a knee and an ankle of the leg, and (b) (i) a portion of the transmitting coil that is disposed over the receiving coil is closer to the ankle than (ii) a portion of the transmitting coil that is disposed at 180 degrees from the portion of the transmitting coil that is disposed over the receiving coil, is to the ankle.
 4. The method according to claim 1, wherein placing comprises placing the housing such that the first distance is 15-45 mm.
 5. The method according to claim 1, wherein placing comprises placing the housing such that the second distance is less than 30 mm.
 6. The method according to claim 1, wherein placing comprises placing the housing such that the third distance is 40-60 mm.
 7. The method according to claim 1, wherein placing comprises placing the housing such that a difference between the third distance and the second distance is 30-40 mm.
 8. The method according to claim 1, wherein providing the transmitting coil comprises providing a transmitting coil wherein a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is greater than 0.5.
 9. The method according to claim 1, wherein providing the transmitting coil comprises providing a transmitting coil wherein a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is less than 1.5.
 10. The method according to claim 1, wherein providing the transmitting coil comprises providing a transmitting coil wherein a ratio of (a) a difference between the third distance and the second distance, to (b) a longitudinal length of the receiving coil is between 0.5 and 1.5.
 11. The method according to claim 1, wherein providing the transmitting coil comprises providing a transmitting coil wherein: (a) a height of the transmitting coil measured along a longitudinal axis of the transmitting coil is 300-600 microns, (b) an outer diameter of the transmitting coil is 100-140 mm, and (c) a ratio of the outer diameter of the transmitting coil to the height of the transmitting coil is at least
 150. 12. The method according to claim 1, wherein placing comprises placing the housing such that the transmitting coil is over a receiving coil, wherein: (a) a longitudinal length of the receiving coil is 3-15 mm, (b) an outer diameter of the receiving coil is 0.6-1.5 mm, and (c) a ratio of the outer diameter of the receiving coil to the longitudinal length of the receiving coil is less than 0.5.
 13. (canceled)
 14. The method according to claim 1, wherein placing comprises placing the housing against the skin and subsequently sliding it along the skin until an indicator, coupled to the housing, indicates that the transmitting coil is in an acceptable position with respect to the receiving coil.
 15. The method according to claim 1, wherein providing the transmitting coil comprises providing a transmitting coil wherein a cross-sectional area of a wire of the transmitting coil is rectangular, wherein the cross-section is taken perpendicular to a direction of current flow within the wire.
 16. The method according to claim 1, wherein placing comprises placing the housing against the skin such that the transmitting coil is elongated in a direction perpendicular to the central longitudinal axis of the receiving coil.
 17. The method according to claim 1, wherein providing the transmitting coil comprises providing a planar coil disposed in a housing.
 18. The method according to claim 17, wherein providing the planar coil comprises providing a planar coil comprising a plurality of layers.
 19. The method according to claim 18, wherein providing the planar coil comprises providing a planar coil with a line spacing, of adjacent coplanar wires, of 0.25-3 mm.
 20. The method according to claim 18, wherein providing the planar coil comprises providing a planar coil with a line width of 1-4 mm.
 21. The method according to claim 1, wherein providing the transmitting coil comprises providing a transmitting coil wherein an average distance from a wire of the transmitting coil to the central longitudinal axis of the transmitting coil is less than two times a square root of a cross-sectional area of a central non-coiled region of the transmitting coil.
 22. The method according to claim 21, wherein providing comprises providing a transmitting coil wherein an average distance from the wire of the transmitting coil to the central longitudinal axis of the transmitting coil is 0.6-1.5 times the square root of the cross-sectional area of the central non-coiled region of the transmitting coil. 23-102. (canceled) 